(1) Field of the Invention
The present invention relates to a multi-channel fiber optic rotary joint device for passing signals on multiple, single-mode or multi-mode optical fiber channels across a continuous rotary interface with low loss, low crosstalk between channels. Light may travel in either direction through the rotary joint device and the interface may be rotated continuously in either direction. The rotary joint device of the present invention has utility in systems having a rotary component such as in fiber links to rotating antennas and winches for a towed buoy.
(2) Description of the Prior Art
Multiple channel rotary joints for optical fibers have been developed and are known in the prior art. U.S. Pat. No. 4,725,116 to Spencer et al. illustrates one such rotary joint which can simultaneously transmit optical signals along two or more channels in either of two directions. The rotary joint has a rotor and a stator to each of which is connected a plurality of optical fibers. Within the joint, reflecting mirrors are used to redirect off-axis optical signals onto the joint axis with relative rotation occurring while the signals are on-axis. A rotating member for each channel has a mirror for reflecting the on-axis signal portion off-axis to a receptor fiber. Alignment between the rotating member and the receptor fiber as well as the drive for the rotating member is provided by a pair of magnets of opposite polarity. One of the magnets is secured to the rotating member while the other is secured to the rotor. The magnetic interaction is intended to insure synchronous rotation of the rotor and the rotating member.
Another rotary joint is illustrated in U.S. Pat. No. 4,900,117 to Chen. This joint or coupler comprises a rotor and a stator joined together by a set of bearings. The two parts are similarly constructed. Each part has circular tracks of selectively angled reflectors for projecting light beams to corresponding receptors during relative rotation with the other part to achieve optical links.
Still another type of rotary joint is exemplified by U.S. Pat. No. 4,934,783 to Jacobson. The Jacobson rotary joint includes a rotating input lens assembly which emits a ray set from a focal point to illuminate a cylindrical reflector. Rays reflected from the cylindrical reflector pass through another focal point and are reflected from the cylindrical reflector to pass through still another focal point. This reflection and focusing continues until the ray paths are incident to a window in the cylindrical reflector and exit. Exiting rays are focused by a lens arrangement which focuses the rays to an output optical fiber
Yet another type of rotary joint, which is a predecessor to the present invention, is exemplified by U.S. Pat. No. 4,109,998 to Iverson. The Iverson rotary joint utilizes a derotating optical element such as a dove prism to derotate the images of an input set of optical transmitters located on the rotor, so that they may be focused onto stationary photo detectors located on the stator. Derotation is accomplished by gearing the rotor and prism in such a way that the prism rotates half as fast as the rotor. The Iverson optical rotary joint does not utilize optical fiber, but rather light emitting diodes or lasers and detectors. As a result, it does not require the high alignment accuracy required for single-mode optical fibers, because the detectors may be quite large. In consequence, the device is not bidirectional and cannot be used as a passive fiber optic device. Because of the relatively large size of the detectors, they will be limited in bandwidth to well below the bandwidth capability of the optical fiber. Several design features preclude its adaptation to single-mode fiber. The lenses which focus light from input to output are large lenses located on the prism rotor on either side of the prism. While acceptable for the relatively crude alignment of LEDs and detectors as in the Iverson patent, this would be limited for a single-mode fiber device by the requirement to maintain ultra-tight lateral and angular tolerances between rotor, prism rotor, and stator. Additionally, some channels are necessarily off the lens axis, leading to unacceptable aberrations in coupling a plurality of single-mode fibers. In general, because it was not required, the Iverson patent does not even discuss the associated mechanical means of meeting and maintaining the critical alignment tolerances imposed by single-mode fiber.
Some multiple channel rotary joints for multi-mode fibers exhibit high optical loss and variation of loss with rotation. Additionally, the extreme tolerances associated with single-mode fibers preclude the easy adaptation of these devices to handle single-mode fibers. The tight tolerances result in single-mode devices with excessive loss or excessive variation of loss with rotation. Rotary joints for single-mode fibers have been developed but are limited to a single on-axis fiber. Devices which actively align the output fibers to track the input fibers have been discussed but these are more complicated, require electrical power, and do not allow operation in both directions.
Accordingly, it is an object of the present invention to provide a multi-channel fiber optic rotary joint device which avoids the aforementioned problems in a bidirectional manner. In contrast to the U.S. Pat. No. 4,109,998 to Iverson (discussed above), the present invention is a passive optical device which is not limited in bandwidth except as the optical fiber is so limited.
It is a further object of the present invention to provide a fiber optic rotary joint device which allows large numbers of fibers to be passed.
It is yet another object of the present invention to provide a fiber optic rotary joint device which allows multiple fiber performance with single-mode fibers.
It is still another object of the present invention to provide a fiber optic rotary joint device having an improved bearing system for permitting improved alignment of the rotor.
It is still another object of the present invention to provide a fiber optic rotary joint device which can be easily aligned with superior accuracy.
Still other objects and advantages of the present invention will become more apparent from the following description and drawings in which like reference numerals depict like elements.